The present invention relates to a process for producing a purified conjugated diene, which comprises a step of isolating a conjugated diene from a petroleum fraction containing the conjugated diene, such as a C4 hydrocarbon fraction or C5 hydrocarbon fraction, by extractive distillation, and more specifically to a process for producing a purified conjugated diene while inhibiting the formation of polymers in the interior of a purification apparatus upon isolation and purification of a highly purified conjugated diene such as isoprene or butadiene from a petroleum fraction.
According to the production process of the present invention, the polymerization of a conjugated diene is prevented in the isolation and purification process of the conjugated diene from the petroleum fraction, thereby inhibiting the formation of popcorn polymers and rubbery polymers in the purification apparatus and preventing a stain on a distillation column and a heat exchanger, and in its turn making it difficult to cause clogging of piping and reduction of thermal efficiency.
Conjugated dienes such as 1,3-butadiene, isoprene and chloroprene are easy to accidentally form porous insoluble polymers (the so-called popcorn polymers) and rubbery polymers. In particular, the recovering or purifying step of a conjugated diene by industrial distillation has various conditions liable to induce polymerization, such as proper processing temperature, high monomer purity, coexistence of gas and liquid phases, mixing of water and presence of iron rust.
The porous insoluble polymers are crosslinked, solvent-insoluble polymers and called popcorn polymers by reason of their external appearance. Once a popcorn polymer is formed, it becomes a seed to self-multiply, so to speak, exponentially in the presence of the vapor and liquid of a conjugated diene, whereby the interior of the apparatus is rapidly clogged therewith. Since the popcorn polymer is a tough, crosslinked polymer, it is insoluble in known solvents and moreover not melted even when it is heated. Accordingly, the popcorn polymer is extremely difficult to remove.
For the removal of the popcorn polymer, there is no effective removing method except cleaning by a mechanical means. In order to conduct the cleaning, it is necessary to stop the apparatus for a while to disassemble it and mechanically remove the polymer deposited on each part. Therefore, it takes much time, and so this method cannot escape an economical disadvantage. In addition, the popcorn polymer cannot be completely removed by the mechanical cleaning, and so the multiplication of the popcorn polymer restart from a seed which is a trace amount of the popcorn polymer remaining in the interior of the apparatus after operation is resumed. The rubbery polymers adhere to devices such as distillation columns, heat exchangers and piping to stain these devices.
On the other hand, a conjugated diene such as 1,3-butadiene or isoprene is generally isolated and purified from a petroleum fraction containing the conjugated diene, such as a C4 petroleum fraction or C5 petroleum fraction by a series of distilling operations containing extractive distillation. Since many hydrocarbons having similar boiling points to each other are contained in the conjugated diene-containing petroleum fraction, the conjugated diene cannot be isolated and purified by only a distilling process making use of a difference in boiling point. In the isolation and purification process of the conjugated diene from the petroleum fraction, a step of isolating the conjugated diene by extractive distillation making use of a difference in solubility in a solvent is thus provided.
However, even in the isolation and purification process of the conjugated diene including the step of extractive distillation, such popcorn polymers and rubbery polymers as described above are easy to form, and so such a process has also involved a problem that the polymers stain or clog devices, for example, extractive distillation columns, distillation columns, heat exchangers, reflux condensers, evaporators, etc.
In order to prevent the polymerization of a conjugated diene-containing petroleum fraction in a distillation apparatus, it has heretofore been proposed to distill a C5 petroleum fraction in the presence of a di-lower alkyl-hydroxylamine (N,N-dialkylhydroxylamine) (Japanese Patent Application Laid-Open No. 112304/1975). According to this method, it is said that purified isoprene can be recovered while inhibiting the formation of a popcorn polymer in a distilling step.
As a method of purifying isoprene or butadiene, Japanese Patent Publication Nos. 41323/1972 and 19682/1970 disclose a method in which a hydrocarbon mixture containing a conjugated diene is subjected to extractive distillation using an extraction solvent containing a polymerization inhibitor or chain transfer agent.
Japanese Patent Application Laid-Open No. 81526/1981 and Japanese Patent Publication No. 20281/1968 disclose a method in which a conjugated diene-containing petroleum fraction is subjected to extractive distillation using an extraction solvent containing furfural and a polycondensate of furfural.
However, the conventional methods in which the polymerization inhibitor or chain transfer agent is only caused to exist in the extraction solvent in the extractive distillation of the conjugated diene-containing petroleum fraction fail to prevent the formation of polymers in the distillation apparatus over a long period of time. Therefore, popcorn polymers and rubbery polymers are formed during the operation to stain or clog the distillation apparatus. It is difficult to prevent the formation of the polymers in, particularly, a reflux condenser and an evaporator over a long period of time, so that such a method has involved a problem that piping is clogged, or thermal efficiency in condensation or evaporation is reduced.
It is an object of the present invention to provide a process for producing a purified conjugated diene, comprising a step of isolating a conjugated diene from a petroleum fraction containing the conjugated diene by extractive distillation, by which the formation of popcorn polymers and rubbery polymers can be inhibited over a long period of time.
Another object of the present invention is to provide a process for producing a purified conjugated diene, by which the polymerization of a conjugated diene in an extractive distillation apparatus can be prevented, thereby inhibiting the formation of polymers, a stain on the interior of the apparatus, clogging of piping, reduction in thermal efficiency, etc.
By the way, in techniques that a conjugated diene is isolated from a petroleum fraction containing the conjugated diene by extractive distillation using an amide compound as an extractive solvent, it has heretofore been considered that to prevent penetration of water into the system to the utmost so as to operate the distillation in an non-aqueous state is a preferred process for preventing the formation of polymers and the corrosion of the apparatus.
The present inventors have carried out an extensive investigation repeatedly with a view toward achieving the above objects. As a result, it has been found on the contrary that in a process for producing a purified conjugated diene, comprising a step of isolating a conjugated diene from a petroleum fraction containing the conjugated diene by extractive distillation, an amide compound is used as an extraction solvent, and water is caused to exist in a specific proportion in the extraction solvent composed of an amide compound, whereby the polymerization of the conjugated diene can be prevented.
According to the process of the present invention, the formation of popcorn polymers and rubbery polymers can be inhibited even in a long-term operation to prevent a stain on the distillation apparatus and clogging of piping. In addition, since the concentration of water in the extraction solvent is adjusted within a specific range in the process of the present invention, the corrosion of the distillation apparatus is not facilitated. The present invention has been led to completion on the basis of these findings.
According to the present invention, there is thus provided a process for producing a purified conjugated diene, comprising a step of isolating a conjugated diene from a petroleum fraction containing the conjugated diene by extractive distillation, the step comprising:
(1) using an amide compound as an extraction solvent,
(2) feeding the extraction solvent to an extractive distillation column, said extraction solvent having been adjusted in such a manner that water is present within a range of 50 to 1,000 ppm based on the extraction solvent, and
(3) subjecting the petroleum fraction to extractive distillation within the extractive distillation column.